This invention relates generally to a catalyst for the production of phthalic anhydride from the oxidative treatment of o-xylene with air. More specifically, the catalyst comprises a poreless, mechanically-stable, temperature-resistant, inert carrier to which there is adhered a catalytically active surface coating.
Supported catalysts containing vanadium pentoxide and titanium dioxide in the catalytically active mass have been known for a long time and are used in technical processes for the production of phthalic anhydride. As promoters or inhibitors to this vanadium pentoxide-titanium dioxide mass, nearly all of the oxide forming elements alone, or in different combinations and varying mixing rates, have been described. See for example DOS Nos. 2547624, 2510994 and 2106796.
One disadvantage of existing industrial catalysts is, that to obtain a high yield and a satisfactory quality of phthalic anhydride under commercial operating conditions, the presence of sulfur or sulfur dioxide is required in the feedstock. If not otherwise present, it has to be added to the ortho-xylene air mixture prior to reaction. Most commercial prior art operations require on the order of 0.1 to 0.3% S or 0.2 to 0.6% SO.sub.2, by weight, based upon the ortho-xylene used. But the presence of the sulfur compounds gives rise to environmental pollution problems, requiring extra steps to de-sulfurize one or more effluent streams, before release. The alternative, of course, is to pollute the environment, either air or water, with malodorous sulfur compounds. It is, of course, very well known that even trace amounts of sulfur compounds are easily detectable by the senses and are harmful. Hence, prior art systems that require addition of sulfur compounds are inherently undesirable.
It is commonly known that the activity and selectivity of a catalyst is closely related, but to date not predictable from its composition, structure, and method of preparation.